This invention relates to the production of a substitute natural gas (SNG) from hydrocarbon oils derived from crude oil, oil shale, coal or other fossil fuels. The process of the invention involves the hydrogenation of a substantial proportion of the oil, suitably using either the non-catalytic processes of fluidised bed hydrogenation (FBH) or gas recycle hydrogenation (GRH). Both FBH and GRH are per se well known, and are described, for example in our United Kingdom patent specification Nos. 1219916, 119909, 11543211, 1133483, 1122426, 1085613, 1074932, 1031717, 1036890, 1031674, 899574, 873832 and 830960.
Natural gas is a highly desirable fuel because of its freedom from sulphur and its clean burning characteristics. Sulphur in certain fuels is a prime source of air pollution, since it produces noxious sulphur dioxide upon combustion. In view of the desirability of consuming natural gas for ecological and other reasons, it has been projected that there will be a shortage of this fuel in the future since its supply is, of course, limited. High quality natural gas has a calorific value of about 1000 Btu/standard cubic foot (BTU/scf). Various proposals have been made in recent years for providing a substitute natural gas, i.e. a manufactured gaseous fuel, which is completely interchangeable with natural gas. For example, U.S. Pat. No. 3,732,085 describes a process for converting crude oil to SNG by means of catalytic hydrocracking. However, the processes, both catalytic and non-catalytic, which have been proposed have limited application either due to their low thermal efficiencies or their restricted range of suitable feedstocks.
Generally, a wider range of feedstocks can be gasified in non-catalytic hydrogenation processes compared with catalytic hydrogenation processes because in the latter the catalysts are deactivated by materials in the feedstock such as metals, asphaltenes and resins. On the other hand, the use of non-catalytic processes can lead to relatively low thermal efficiencies due to the need to produce large amounts of hydrogen. In these schemes it is normally desirable to provide the required hydrogen by partial oxidation (POX), a process with a relatively low efficiency, so as to consume certain heavy fractions of the oil feedstock and oils produced from other process units. The purpose of this is to obtain complete feedstock utilization, allowing the lighter fractions to be gasified to methane.
Under comparable conditions, the ratio of hydrogen to oil feedstock fed to a thermal hydrogenator determines the amount of feedstock gasified, and to achieve the required degree of gasification it is necessary to supply hydrogenators like the FBH and GRH with a surplus of hydrogen over that required for reaction. The unreacted hydrogen has to be eliminated from the hydrogenated gas since it is unacceptable, except in small quantities, in SNG. Processes have been proposed in which this hydrogen is reacted usually in the presence of a catalyst, with carbon oxides and ethane in the hydrogenated gas to give methane. These processes are not very attractive for two reasons. Firstly, the reaction of carbon oxides and hydrogen (methanation) is very exothermic and must be conducted at low temperatures so that much low grade heat is released which is not readily recovered in a useful way. Secondly, the reaction of ethane and hydrogen suffers from the further disadvantage that the ethane is not then available to enrich the final gas to the calorific value (CV) specified for SNG. This is particularly disadvantageous in those situations where the specified CV is higher than that of pure methane, such is the case in the United Kingdom and Japan. It is thus an object of the present invention to reduce the amount of hydrogen produced and to conserve at least part of the ethane produced in the hydrogenator. It is another object of the present invention to provide such a process in which more efficient use is made of the hydrogen produced than hitherto.